let a = [1, 2, 3];
console.log(a.concat(4, 5)); // [1,2,3,4,5]
console.log(a.concat([4, 5], [6, 7])); // [1,2,3,4,5,6,7]
console.log(a.concat(4, 5, [6, 7])); // [1,2,3,4,5,6,7]
console.log(a.concat(4, [5, [6, 7]])); // [1,2,3,4,5,[6,7]]
// a 保持不变
console.log(a); // [1,2,3]
